The present disclosure relates to an apparatus for producing glass products from a glass melt while avoiding bubble formation, according to the preamble of apparatus claim 1, as well as a method which can be implemented with the apparatus according to the preamble of the independent method claim. In particular, the present disclosure relates to an apparatus and a method for the production of glass products, such as tube glass, from a glass melt which is conditioned, in particular homogenized, in an electrically heated crucible by means of a component located and/or mounted therein. Such a crucible can be, for example, a stirring crucible in which a stirring element is mounted so as to be rotatable as a component. For example, it can also be a crucible in which a tube is provided as a component in order to produce, for example, tubular glass according to the Vello method. The present disclosure is, however, intended to be applicable to any type of crucible and a component located therein.
For the production of glass products, apparatuses are used, which are also referred to as glass melting units and, more generally, also as production units. During production, the formation of bubbles in the respective glass melting unit is to be minimized or, if possible, completely prevented.
A typical glass melting unit comprises the area from the insert of the batch with the inserting machines to the outlet at which the still liquid glass is transferred to the post-processing devices (for shaping the desired glass product). These can be, for example, drip feeders, roller machines, pipe drawing plants or else a tin bath, onto which the liquid glass runs.
Glass melting furnaces generally consist of a region which is used for the formation of the liquid glass melt, a region in which the melt is refined, wherein the residual gas bubbles remaining after the melting process being removed from the melt, and a region which is used for conditioning. In this last region, the melt is generally still homogenized and brought to the temperatures necessary for the post-processing. The temperatures required for the entire melting process are strongly dependent on the type of glass. For example, soda-lime glasses, which are used, for example, for the production of window glass and glass containers, are melted at significantly lower temperatures than, for example, special glasses for display present disclosures or glass ceramics.
The melting tank can be purely fossil-heated, be provided with an additional electric heating or can also be operated completely fully electrically. As a combustion medium for the gas burners, which is usually preheated, normal air or even pure oxygen can be used.
The melting and refining zones can be located in a common basin or also in spatially separate sections. Various installations for influencing the flow are also state of the art.
The conditioning step is carried out in the working tank or in a gutter and distributor system. The latter can be constructed in a variety of ways, wherein it can contain one or more metallic components or installations. Stirring elements (stirrers, stirring needles, etc.) are often used for homogenization, which are stirring the glass melt located in a stirrer.
It is well-known that bubbles can merge or form in the glass melt in electrically heated glass melting units, such as in stirrers, which ultimately result in a significant deterioration in the product quality and, therefore, the formation of these should be completely suppressed.
DE 10 2010 037 376 A1 discloses a method and an apparatus for the production of glass products, with essential restrictions being placed on the electrical heating of glass melts. The formation of bubbles at and in the vicinity of the heating electrodes or of the current supply flanges caused by excessively high current loads can be positively influenced by the heating frequency used, the electrode materials used, and the properties of the glass itself.
Recommendations are given for the limitation of the current load to be adhered to different redox systems.
In DE 199 55 827 A1 a method and an apparatus are disclosed to improve the glass quality, wherein the (electroless) O2 bubble formation is avoided due to the decomposition of water in that a DC voltage source is contacted to a conductive part (e.g. wall) of the aggregate and to the electrode protruding into the molten glass in order to produce a potential gradient in the glass melt, which causes the oxygen remaining after migration of the hydrogen to be ionized, i.e. is converted to negatively charged O2 ions, which is—in contrast to neutral O2 molecules—dissolve in an unlimited amount in the melt and thus do not agglomerate to gas bubbles.
Other methods and apparatuses for producing glass products while avoiding gas bubble formation are disclosed in the following documents:
DE 10 2008 042 117 A1, JP H 09 67 127 A and JP 2005 060 215 A.
Accordingly, the known methods and apparatuses either deal primarily with the problem of (electroless) gas bubble formation due to water decomposition or with the problem of (current-conditioned) gas bubble formation due to electrical alternating current heating. However, it would be a solution that would address and solve both problems in an efficient manner.
The object of the present disclosure is to further improve a method and an apparatus for the production of glass products of the type mentioned above so that both electroless and current-conditioned bubble formations are effectively prevented.
The object is achieved by an apparatus with the features of method claim 1 as well as by a method with the features of the independent apparatus claim.
Accordingly, the present disclosure relates to an apparatus which comprises a crucible, such as a stirring crucible, and a component located therein such as, for example, a rotatably mounted stirrer, and which comprises an alternating current unit for heating the molten glass, which feeds the crucible with current via current connection elements, wherein the component or stirrer is now connected to one of the current connection elements (e.g. lower heating flange) of the crucible or agitation bar via a current-limiting choke, which has a variable impedance. By means of such a current-limiting choke, by adjusting the impedance, it can be achieved simultaneously that the AC current load of the apparatus is minimized and that the water decomposition reaction is positively influenced.
The present disclosure applies to all types of crucibles and components located therein. If, for example, a stirring crucible or bar is provided with a stirrer, the stirrer is preferably connected directly to a heating flange by means of a sliding contact and an external cable. This changes the electric field and also the current density distribution in the apparatus. Practical tests have shown that the connection with the lower heating flange has the greatest effect on the bubble quality. Often, this effect was only a short time, so the connection was disconnected again. As a rule, any change, i.e. the clamping or disconnecting of the sliding contact, has already led to a reduction in the occurrence of bubbles and thus to an improvement in the product quality. Therefore, a reversing circuit of the sliding contact could be implemented with the possibility of presetting a time duration.
The present disclosure proceeds from the following surprising finding:
In order to reduce the current-induced bubble formation, it would be an easy to implement measure to reduce the alternating current and thus the current density as much as possible. Although this would avoid a high current density on the stirrer flights and the associated bubble formation, however, an alternating current which is too low might not be sufficient to form an effective direct current buffer for the water decomposition reactions on the conductive walls of the stirring crucible. Therefore, by means of the current-limiting choke proposed here, it is caused by setting the impedance to an optimum value for the respective configuration, a minimum bubble formation occurs so that the product quality can be optimized in a simple and very effective manner. Experiments have shown, that surprisingly the bubble occurrence reaches a minimum at a given impedance ZD of the choke, i.e. that too small AC current or an excessively high AC current in this branch leads to more bubbles in the product. In an embodiment of the present disclosure, the impedance of the choke or inductor can be adjusted in a simple manner via corresponding taps and via a resistor, if the current-limiting choke is implemented via a heating transformer. The secondary side or the primary side of the transformer can be used for this purpose. It is important to pay attention to the ratio of the transformer. The principle of the present disclosure and its effects are described in more detail below. For this, preferred exemplary embodiments are disclosed, which also result from the subclaims.
Accordingly, the stirring crucible is preferably made from a precious or noble metal or metal alloy, in particular from a Pt—Rh alloy, and a first heating flange mounted on an upper end of the stirring crucible and a second heating flange mounted at a lower end of the stirring crucible serve as current connection elements. Preferably, the stirrer element is connected to the second heating flange (at the lower end of the stirrer) via the current-limiting choke. The stirrer element(s) can be, for example, a stirrer or one or more stirring needles.
In a preferred embodiment of the present disclosure, the apparatus has a contact connection element, in particular a contacting flange, which contacts the current-limiting choke with the stirring crucible in an alternative manner to the contact via the current connection element. This additional contacting flange is not used for heating purposes but only for contacting the stirrer via the choke with the crucible.
The variable impedance of the current-limiting choke is preferably adjusted in such a way that an alternating current density occurring in the glass melt, in particular at the ends of stirrer wings or vanes, lies between a lower limit value and an upper limit value. In this respect, the lower limit value indicates a minimum required alternating current (AC) current density which provides on the inner wall of the stirring crucible a sufficient direct current (DC) buffer for preventing from water decomposition reactions with bubble formation. The upper limit value, in turn, specifies a maximum permissible AC density, above which bubbles are formed in the glass melt, in particular on stirrer blade ends and/or in the region of the current connection elements. With the adjusted impedance value one can thus find the optimum for AC load and DC protection.
In particular, the impedance of the current-limiting choke can also be set as a function of at least one of the following parameters:                frequency of a heating current generated by the alternating current generator;        material of the crucible, in particular of the stirrer of the stirring element and/or of the current connection elements;        geometry including the material thickness of the crucible, in particular of the stirrer of the stirring element and/or of the current connection elements;        impedances of the current connection elements of the contact connection element and/or of their supply lines;        the type of glass used, in particular the type and quantity of redox elements used in the glass melt.        